1. Field of the Invention
The present invention generally relates to semiconductor devices and methods of producing the same, and more particularly, to a semiconductor device having a semiconductor chip mounted on a substrate in a face-down manner and a method of producing the same.
For example, a semiconductor device having a BGA (Ball Grid Array) structure is provided with a semiconductor chip mounted and resin-sealed on an upper surface of a substrate and with solder balls serving as mounting terminals on a bottom surface of the substrate. As a method of mounting this semiconductor chip on the substrate, a wire bonding, by means of which the semiconductor chip is electrically bonded to the substrate in a face-up manner, and a face-down bonding, by means of which the semiconductor chip is mounted on the substrate in the face-down manner, are known.
Particularly, the face-down bonding can contribute to an improvement in the high frequency characteristic and the miniaturization of a semiconductor device since the face-down bonding requires neither wires nor a space for disposing the wires in the semiconductor device. On the other hand, the face-down bonding should be performed with reliability high enough to support the increasing number of the terminals of semiconductor chips with high density.
2. Description of the Related Art
FIG. 1 is a flowchart showing a conventional method of producing a semiconductor device wherein the face-down bonding is employed to mount a semiconductor chip having solder bumps on a substrate.
This method of producing the semiconductor device proceeds as follows.
First, the solder bumps are formed on an active circuit surface of the semiconductor chip by an evaporation method or the like, and flux is applied to bonding pads of the substrate. Then, at step S10 (the word xe2x80x9cstepxe2x80x9d is simply referred to as xe2x80x9cSxe2x80x9d in the diagrams), the semiconductor chip is bonded to the substrate in the face-down manner so that each of the solder bumps will be mounted on the corresponding bonding pad. Thus, the semiconductor chip is temporarily mounted on the substrate by the flux.
Next, a reflow soldering process is performed at step S11, wherein the semiconductor chip temporarily mounted on the substrate by the flux is put in a reflow furnace so that the solder bumps of the semiconductor chip will be melted and fixed to the corresponding bonding pads of the substrate. After this reflow soldering process, a cleaning process to remove residual flux and a drying process are performed at step S12 so that the semiconductor chip will be fixed to the substrate.
At step S13, an underfill resin made of, for example, an epoxy resin or the like is interposed between the semiconductor chip and the substrate, and next, at step S14, a thermosetting process to harden this underfill resin is performed.
Further, at step S15, solder balls are attached to corresponding external terminals formed on a bottom surface (a surface opposite to the surface on which the semiconductor chip is mounted) of the substrate, and finally, at step S16, another reflow soldering process is performed so that the solder balls will be melted and joined to the corresponding external terminals.
Conventionally, the semiconductor devices having the solder bumps for use as bumps have been produced in the above described method including steps S10 through S16.
On the other hand, FIG. 2 is a flowchart showing a conventional method of producing a semiconductor device wherein the face-down bonding is employed to mount a semiconductor chip having gold bumps on a substrate.
This method of producing the semiconductor device proceeds as follows.
First, at step S20, a conductive resin such as silver paste is applied to a top end of each of the gold bumps, and next, at step S21, the semiconductor chip is bonded to the substrate in the face-down manner with heat and load being applied thereto.
Then, at step S22 after the semiconductor chip is bonded to the substrate in the face-down manner at step S21, an underfill resin made of, for example, an epoxy resin or the like is interposed between the semiconductor chip and the substrate. Thereafter, at step S23, a thermosetting process to harden this underfill resin is performed.
Further, as in the method shown in FIG. 1, solder balls are attached to corresponding external terminals formed on a bottom surface of the substrate at step S24, and are joined to the corresponding external terminals by a reflow soldering at step S25. The semiconductor devices having the gold bumps for use as bumps have been produced in the above described method including steps S20 through S25.
Moreover, Japanese Laid-Open Patent Application No. 10-303252 discloses a semiconductor device having a reinforced connection between pad electrodes and bump electrodes. In this semiconductor device, a convex portion having a smaller diameter than the bump electrode of the semiconductor chip is formed on each of the pad electrodes, and is pressed into the corresponding bump electrode so as to realize the reinforced connection therebetween.
However, according to the conventional production method as shown in FIG. 1, wherein solder is employed as material for the bumps formed on the semiconductor chip, time for melting the solder bumps and adhering the melted solder bumps to the substrate is necessary, and further, a reflow process is required so as to keep the steady figures of the solder bumps.
Moreover, since flux is employed in the temporary mounting of the solder bumps on the corresponding bonding pads of the substrate, a cleaning process to remove residual flux is needed after the reflow process. For the above reasons, this production method has been precluded from achieving efficiency and thus has required a long time in producing semiconductor devices, entailing the problem of a rise in the production cost thereof.
Further, the conventional production method as shown in FIG. 2, wherein the semiconductor chip is bonded to the substrate by means of the gold bumps having silver paste on the top ends thereof, has also taken a long time in producing semiconductor devices because the silver paste requires time for applying heat thereto so as to be hardened when the semiconductor chip is bonded to the substrate.
In addition, as a semiconductor chip has higher density, the number of bumps formed on the semiconductor chip is on the rise with the result that the individual bumps become smaller in size. The downsizing of the individual bumps makes it difficult to securely interpose an underfill resin between the semiconductor chip and the substrate as the space therebetween becomes narrower when the semiconductor chip is mounted on the substrate. Unless the underfill resin is securely interposed therebetween, there rises the problem of the lowered yield rate and reliability of the semiconductor device since a stress resulting from a difference in coefficient of thermal expansion between the semiconductor chip and the substrate is applied to the bumps, so that the bumps are detached from the semiconductor chip.
It is a general object of the present invention to provide a semiconductor device and a method of producing the same in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a semiconductor device having a greater reliability in the connections between bumps and protruding electrodes and a method of producing such a semiconductor device with more efficiency.
The above objects of the present invention are achieved by a semiconductor device including a substrate having a first surface on which a plurality of protruding electrodes are protrudingly formed in correspondence to an arrangement of a plurality of bumps formed on a semiconductor element, and a second surface on which balls serving as mounting terminals are formed, the semiconductor element being bonded in a face-down manner to the substrate with the protruding electrodes being embedded into the bumps, and alloy layers having materials identical to those of the bumps and the protruding electrodes formed on interfaces of the bumps and the protruding electrodes.
By embedding the protruding electrodes into the bumps, the protruding electrodes break through oxide films to reach inside the bumps even though the oxide films are formed to cover the surfaces of the bumps. Therefore, the electrical connections between the bumps and the protruding electrodes can be prevented from being deteriorated by the oxide films. Further, the alloy layers having the materials identical to those of the bumps and the protruding electrodes formed on the interfaces therebetween can enhance both of the mechanical and the electrical connections therebetween.
The above objects of the present invention are also achieved by a method of producing a semiconductor device including the steps of embedding protruding electrodes formed on a first surface of a substrate into bumps formed on a semiconductor element, interposing an insulating resin between the semiconductor element and the substrate, and applying heat having temperature equal to or higher than respective melting points of the bumps and balls serving as mounting terminals formed on a second surface of the substrate so as to form alloy layers on interfaces of the bumps and the protruding electrodes and simultaneously to form the balls on the second surface of the substrate.
In the embedding step, the protruding electrodes formed on the substrate are embedded into the bumps formed on the semiconductor element. At this point, no heat treatment or the like is provided, and the protruding electrodes are mechanically embedded into the bumps by simply applying a pressing force (load) so that the bumps and the protruding electrodes will temporarily be joined. Thus, as the semiconductor element is bonded to the substrate in the face-down manner without requiring time for the heat treatment, the face-down bonding can be performed in a short time.
Further, by interposing the insulating resin between the semiconductor element and the substrate in the resin-interposing step, the semiconductor element is fixed to the substrate and the connecting positions of the protruding electrodes and the bumps are also reinforced by the insulating resin.
Moreover, applying heat having temperature equal to or higher than respective melting points of the bumps and the balls in the heat application step so as to form the alloy layers on the interfaces of the bumps and the protruding electrodes and simultaneously to form the balls on the substrate can save time and work in the whole production process and reduce the production costs so that semiconductor devices with high performance can be supplied to the market at inexpensive prices.